CHICAGO — Donna Yehl's 4th grade students bob behind their desks, heads going up and down as if they sat on the deck of a rolling ship.

But it's not what it appears to be, a bad case of fidgeting.

The two-dozen kids in Yehl's classroom at Creekside Elementary School in Elgin, Ill., write and read — in fact, do all of their classwork — perched on exercise balls.

The multicolored inflatable balls are commonly used in pilates, yoga and exercise classes. They're still a rare sight in most classrooms, but teachers increasingly find they provide a multitude of benefits, saying they sharpen attention and improve posture.

At the end of the last school year, Yehl took to the Internet seeking creative ways to help her restless pupils sit still. She stumbled on a story about exercise balls and improved concentration, and opted to replace the kids' metal and plastic chairs with bouncy, 55-centimeter — about 21 inches-high — balls in colors they selected for themselves.

Yehl found herself an almost immediate convert.

"They're more focused," Yehl said, ticking off other benefits she's seen in her two classes that used the balls. "They're sitting upright."

Ball chairs are popping up in classrooms from Pennsylvania to Ontario.

Lisa Witt, whose Wisconsin-based company WittFitt sells exercise ball chairs for classroom use, reports a sharp increase in customers, from just one in 2004 to more than 300 schools across the country and abroad.

"Some people initially think, 'Are you crazy?'" said Witt, a former elementary school teacher in Colorado who started her company after seeing improvements in students who used exercise balls as chairs. She said she soon discovered physical benefits as well as others.

"It's just plain fun," she said.

And the kids often agree.

Emily Ziemba, 10, a student in Yehl's class, laughs and nods when asked if she likes sitting on an exercise ball.

"I mean, sometimes I would like to lean back," she said. "But other times, it's better than a chair."

Teachers say children on ball chairs often quit flopping over on their desks. And although they can't lean far back anymore, they're getting enough exercise to improve concentration.

"You'd be surprised how many kids really need to move while learning," said Adrienne O'Brien, a 4th grade teacher at Roslyn Road School in Barrington, Ill., who has nicknamed her more energetic kids "squirrels." "That would be the majority of them, frankly."